TSB blames optical illusion for plane crash

The fatal crash of a small float plane in central British Columbia was partly the result of an optical illusion associated with low-altitude flights, as well as overweight and improperly secured cargo, the Transportation Safety Board says.

A pilot died and two of four passengers were seriously injured when the de Havilland DHC-2 Beaver float plane went down during a hunting expedition in October 2016 near Laidman Lake, about 150 kilometres southwest of Prince George.

The board’s report describes how the aircraft was more than 300 kilograms over its maximum allowable weight, and because the cargo was unsecured, it hit the rear passenger seats when the plane crashed.

The report also says the pilot likely misjudged how close the plane was to the ground because of an optical illusion associated with flying under an overcast sky and over snow-covered vegetation, causing the aircraft to stall when the pilot tried to avoid hitting the hillside.

“The lack of features to provide scale in the snow-covered terrain, together with the minimal contrast among the dense trees given the diffuse light conditions, likely disguised the upsloping terrain and the actual horizon,” the report says.

“As the slope steepened, the perceived horizon would have moved upward in the windscreen, and the pilot may have pitched the aircraft up to maintain a constant angle between the pilot’s reference point on the aircraft and the rising terrain.”

The two other passengers on board suffered minor injuries during the crash.

The Transportation Safety Board is recommending that all DHC-2 aircraft that operate commercially in Canada be equipped with a system that would issue a warning as an aircraft is about to stall.

“It is reasonable to conclude that the absence of a stall-warning system deprived the pilot of the last line of defence against an aerodynamic stall and the subsequent loss of control of the aircraft.”

The agency says this recommendation would not have helped in this incident because the aircraft was privately operated, but it underscores the benefits of a stall-warning system.

The Transportation Safety Board investigates marine, pipeline, railway and aviation incidents and its purpose is to improve transportation safety, not attribute blame or find fault.